Soil freezing effects:

Transcription

1 Soil freezing effects: Runoff versus infiltration. Root (and tree) mortality. Disruption of nutrient cycles: Increases in N, P, Ca loss. Effects on DOC? Increases in nitrous oxide flux. Decreases in methane uptake. Effects on overwinter CO 2 flux?

2 Winter climate change research at Hubbard Brook (people and funding): Personnel : Peter M. Groffman - Tim Fahey Charley Driscoll - Janet Hardy Melany Fisk - Gene Likens Pam Templer - Myron Mitchell John Campbell - Ross Fitzhugh Natalie Cleavitt - Jason Demers Geri Tierney - Lisa Martel Adam Welman - Annie Socci Lynn Christenson - Rae Melloh Janet Frankenstein - Karen Henry Funding: NSF Ecosystem Studies, LTER

3 Winter climate change research at Hubbard Brook (projects): Long-term USFS interest in snow: 50 year record of soil frost and snow depth, water content and snow cover duration. Days of ice cover for Mirror Lake 49 year record. Excursions in the long-term watershed nitrate export record. Shoveling studies Natural gradient studies

4 MIRROR LAKE

5 Days of Ice Cover Mirror Lake Source: Gene Likens Slope = days/yr R 2 = 0.16 p = Winter Period

6 Max. snow depth (mm) Max. snow water (mm) Snow cover (days) Year Snow course data from Station #2, monitored continuously since Campbell et al. (2007)

7 Hubbard Brook February 6, 2006 Source: Don Buso

8 Monthly Stream Nitrate Concentrations (W6) 1.4 Insect defoliation Nitrate (mg-n L -1 ) Soil frost Ice storm Date Modified from Likens 2004

9 Big ideas: Snow is an insulator of the forest floor. Lack of snow results in soil freezing, root and microbial mortality and accelerated nutrient loss and soil acidification. Climate change could lead to increases in soil freezing episodes. These events occur now.

10 Origins: Long term USFS interest in snow. Mitchell et al Climatic control of nitrate loss from forested watersheds in the northeastern United States. Environ. Sci. Tech. Sugar maple decline in Canada. Work in the western U.S., e.g., Niwot Ridge (Williams, Brooks, et al.), Wyoming (Reiners et al.), Alaska.

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12 Hardy et al. (2001)

13 Soil freezing doubles overwinter root mortality: 0.35 Proportional fine root mortality Reference Treatment Mar-98 May-98 Jul-98 Sep-98 Nov-98 Jan-99 Mar-99 May-99 Jul-99 Sep-99 Date sampled Tierney et al. (2001)

14 Soil freezing increases nitrate leaching: (µmol L -1 ) Soil Solution NO Reference Treatment D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N Oa Horizon Maple 100 Birch D J F M A M J J A S O N D J F M A M J J A S O N b * D J F M A M J J A S O N D J F M A M J J A S O N Frozen Frozen Frozen Frozen Fitzhugh et al. (2001)

15 Soil freezing increases phosphorus leaching: Soil Solution P i (µmol/l) Oa Reference Oa Treatment J F M A M J J A S O N D J F M A M J J A S O J F M A M J J A S O N D J F M A M J J A S O Frozen Sugar Maple Fitzhugh et al. (2001) Frozen

16 Findings: Mild soil frost induces: N, P, H + and base cation leaching (Fitzhugh et al. 2001). Increase in N 2 O emissions (Groffman et al. 2006). Decreases in CH 4 uptake (Groffman et al. 2006). Doubling overwinter root mortality (Tierney et al. 2001). But, some mysteries: Big effects from mild freezing. Field results much more dramatic than lab results. NO change in microbial biomass and activity (including in situ net N mineralization and nitrification (Groffman et al. 2001). Freeze effects dominated by physical/chemical/biological interactions. Can we go to society with results that we don t understand?

17 Nitrate response much less marked in second study: 80 Oa horizon Reference 60 Treatment Soil solution NO 3 - (µmol L -1 ) (pre-treatment) Source: Groffman et al. (2010)

18 Nitrate response much less marked in second study: 80 Bs horizon Reference 60 Treatment Soil solution NO 3 - (µmol L -1 ) (pre-treatment) Source: Groffman et al. (2010)

19 80 Nitrate in Oa horizon ( ) w/dominant tree Treatment Reference NO 3 - concentration (µmoll -1 ) Yellow birch Sugar maple Sugar maple Yellow birch 0 lv uv wk ek Source: Fashu-Kanu/Cleavitt/Martel

20 DOC response may prevent a nitrate response: 800 Oa horizon Reference 600 Treatment Soil solution DOC (µmol L -1 ) ** (pre-treatment) Source: Groffman et al. (2010)

21 DOC response may prevent a nitrate response: 800 Bs horizon Reference 600 Treatment Soil solution DOC (µmol L -1 ) * * (pre-treatment) Source: Groffman et al. (2010)

22 Alteration of flowpaths and abiotic processes also important. THRFALL SNOLYS SOILLYS kg N/ha (Dec 31 - March 22) NO3-N NH4-N Feb Mar Jul 2004 B δ 15 N A A 1 Source: Campbell et al. (2007) 0 Microbial biomass

23 Winter climate change and nitrogen retention States Controllers Biology/C flow Physics/hydrology Plant uptake/next growing season Unfrozen Litter processing /fauna POC/DOC Horizontal effects (lateral flow/runoff Vertical effects (leaching, snowpack interaction with soil profile Overwinter root mortality reduces plant uptake in subsequent growing season Frozen Microbes/ immobilization Freeze/thaw Quantify cumulative overwinter C flux (CO2, DOC, POC 15 N retention in snow lysimeters 15 N plant uptake/ lysimeters

24 The elevation gradient at HBEF: High>780M Mid m ~ 2 o C Low m

25 New winter climate change study: 20 sugar maple sites along elevation gradient: 6 intensive sites (isotope studies), 14 less intensive. Lysimeters, temperature, moisture, frost, snow, microbial biomass and activity. 13 C, 15 N-litter study: Exclusion cores to study roots, microbes, hyphae. Invertebrates.

26 New winter climate change study: 15 N snow lysimeter study: 15 N and Br additions to snowpack. Reciprocal transplants. Flowpath studies : Does soil freezing alter the way water and nitrogen flow across the landscape? Reconcile plot versus watershed scale results. Compare north versus south facing watersheds. Measure solutes that serve as natural tracers of hydrologic flowpaths

27 Summary/conclusions: Soil freezing, which may increase with climate warming, is potentially an important controller of forest biogeochemistry but: Will we have colder soils in a warmer world? What controls the variation in response to soil freezing at the plot scale: Roots? DOC? Plant community composition? How do plot and landscape controls interact?